At present, air conditioning systems used in aircraft for ventilating and pressurising the aircraft passenger cabin and/or the cargo compartment work on the principle of mixing ventilation. In the case of mixing ventilation, which is also called dilution ventilation or jet ventilation, air, which is optionally temperature-controlled as required, is blown into the part of the aircraft to be ventilated, with a high impulse and at a high speed (approximately 0.7 to 3 m/s), through air inlets usually arranged in the ceiling region of a part of the aircraft to be ventilated. Owing to the high impulse and the high speed of the air blown in, tumble air movements form in the part of the aircraft to be ventilated which ensure a virtually homogeneous intermixing of the air with the ambient air, free convection induced by heat sources and cold or hot surfaces being dominated by the forced flow of the tumble air movements. The air jets of the tumble air movements may reach air speeds which are beyond the comfort limit for the passengers and crew. The exhaust air is usually removed from the part of the aircraft to be ventilated through air outlets provided in the floor region of the part of the aircraft to be ventilated. Exhaust air removed through the air outlets is generally at least partly filtered, mixed with fresh outside air in a mixer and finally returned to the part of the aircraft to be ventilated. For setting a desired cabin pressure, as required, excess exhaust air can be removed to the environment through pressure regulating valves (outflow valves) which connect a pressurised region of the aircraft to the outside atmosphere.
The formation of tumble air movements necessarily results in regions in which free convection and forced flow complement one another or are added to one another and regions in which the free convection opposes the forced flow. The resulting ambient flow thus has regions at a high flow speed and regions at a low flow speed. The differences in flow speed increase with increasing heat load. Moreover, the relatively concentrated introduction of the cool air, the jet formation and the inhomogeneous speed and cooling-load introduction in the part of the aircraft to be ventilated may result in an uneven temperature distribution. Furthermore, the supply of very cool air into the passenger cabin leads to marked reductions in the comfort of the passengers and crew members. Consequently, the difference between the temperature of the air supplied via the air inlets and the temperature of the ambient air in the part of the aircraft to be ventilated must not be too great. High heat loads can then, however, only be removed by means of correspondingly high air volume flows which require an air supply at very high speed and/or an enlargement of the air outlets. This may lead to unpleasant draughts and disturbing noises.
The short distance between the air inlets and the seat positions of the passengers in passenger aircraft limits the available jet length and the speed of the air blown in through the air inlets. In certain circumstances, however, air speeds which are at least locally above the comfortable range may result, while at the same time other regions of the cabin are not sufficiently ventilated. Moreover, special regions provided in the aircraft passenger cabin, such as, for example, galleys, rest zones for crew members or bar areas and also curtains used for zoning the aircraft passenger cabin, may disturb the formation of the tumble air movements characteristic of mixing ventilation, which may likewise result in reductions in the comfort of the passengers and the crew members, at least in locally defined regions of the cabin. Furthermore, the partly turbulent air flow conditions arising in mixing ventilation increase the drying-out of the skin, eyes and mucous membranes caused by the low atmospheric humidity in the passenger cabin of only 5 to 15% relative humidity.
A further disadvantage of the mixing ventilation principle is that, due to the tumble air movements which form, harmful substances are rapidly distributed in the entire flow cross-section of the aircraft and, moreover, longitudinal flows are promoted. This is problematical, particularly in the event of a fire with smoke formation, since the smoke is distributed so quickly due to the air flow conditions which arise in mixing ventilation that no uncontaminated escape areas remain.
Finally, an aircraft cabin which is ventilated by an air conditioning system working on the principle of mixing ventilation can be subdivided into individual climatic zones only in the longitudinal direction. This leads to severe restriction of the flexibility in the design of the aircraft cabin which is increasingly demanded by the market.